Of 4H-imidazoles

Oxaziridines, 6-oxa-l,4-diazabicyclo[3.1.0]hex-3-enes (245) and their 4-oxides (246), obtained in the photolysis of 4H -imidazole-3-oxides (223) and 1,3-dioxides (219), underwent a quick thermal isomerization to the starting nitrones. Further... 

Owing to the existence of two centers for nucleophilic attack (at C2 and C5) in radical cations (220) obtained from the oxidation of 4-H -imidazole-1,3-dioxides (219), the formation of two products of methoxy group addition was observed, namely NNR (221) and NR of 3-imidazoline-3-oxide (222). The ratio of the products depends on the electronic nature of substitutes R1 and R2. Both, the donor character of R1 and acceptor character of R2 facilitate the formation of nitroxyl radicals (222) with the yield of (221) increasing with the inverted effect of the substituents. As was mentioned in Section 2.4, the results of preparative electrochemical oxidative methoxylation of 4H -imidazole-1,3-dioxides are similar to the results of chemical oxidation. 

In addition to the oxidative alkoxylation of 4H -imidazole and oxazolidine derivatives, the reaction was also used with other cyclic aldo-nitrones such as DMPO, derivatives of 3-imidazoline-3-oxide (228-232) (506), and derivatives... 

Oxidative Animation of Nitrones to a-Amino-Substituted Nitroxyl Radicals Similar to the oxidative methoxylation reaction, oxidative animation of 4H -imidazole TV-oxides, in amine saturated alcohol solutions, give stable nitroxyl (282), nitronyl nitroxyl (283), imino nitroxyl (284) and (285) radicals with the amino group at the a-carbon atom of the nitroxyl group (Scheme 2.107) (520, 521). The observed influence of substituents on the ratio of animation products at C2 and C5 atom is close to the ratio observed in the previously mentioned oxidative methoxylation reaction. It allows us to draw conclusions about the preference of the radical cation reaction route. 

Recently, a new synthetic route to derivatives of 4H-imidazole was reported (97JPR729). Accordingly, the deeply violet-colored triazafulvalenes 74b and 74c have been synthesized starting from the 2-(4-pyridyl)-substituted compound, followed by quaternization with methyl iodide and subsequent reduction with sodium borohydride (97UP1). Finally, the ring transformation of the preformed oxa-diazafulvalenes as well as their pro-tonated species with primary alkylamines offers a further possibility to synthesize triazafulvalenes (70JCS807) (Scheme 30). 

The oxide function can be removed using phosphorus trichloride, or phosphorus oxychloride, by reduction with hydrogen and Raney nickel" with sodium borohydride, and with hexachlorodisilane in chloroform. The reduction of 4H-imidazole N-oxides with borohydride leads to the 1-hydroxyimidazoline or imidazolidine derivatives under the same conditions 4if-imidazole JV,N -dioxides give 1,3-dihydroxy-imidazolidines. ... 

Maier, G. and Endres, J., Photochemistry of matrix-isolated 4-diazo-4ff-imidazole IR-spectro-scopic identification of 4H-imidazol-4-yhdene, Eur. J. Org. Chem., 2535, 2000. 

D-er7f/zro-pentofuranosyl)amino]-4H-imidazol-4-one (40) [73, 74]. The mechanism of their production (Scheme 4) may be rationalized in terms of the transient formation of oxidizing guanilyl radicals 39 that arise from deprotonation of 38 [75]. The resulting neutral radicals of dGuo(-H) 39 may... 

Unlike the 4H- imidazoles (219), (223), (224) electrochemical oxidation of the nitrone group in 4-R-3-imidazoline-3-oxides (228), (230-232), as in a-PBN and DMPO is of irreversible nature. Therefore, the formation of radical cations... 

Oxidation of mono-iV -oxides 4H -imidazole (223) and (224) with Pb02 in methanol leads to the formation of stable a,a-dimethoxy-substituted nitroxyl radicals (271) and methoxy substituted imino nitroxyl radicals (INR) (272)-(274) (Scheme 2.101) (514). 

Oxidative Fluorination of Nitrones to a-Fluorosubstituted Nitroxyl Radicals Formation of nitroxyl radicals by the radical cation route was observed in reactions of various nitrones with xenon difluoride in dry methylene chloride (520, 523). In this reaction, more than 40 nitrones, including 4H -imidazole N,N -dioxides (219), 4H -imidazole TV-oxides (223) and (224), 2H -imidazole N -oxides (225), 2H -imidazole TV,TV-dioxides (226), 3,3,5,5-tetramethylpyrroline N -oxide (TMPO), derivatives of 3-imidazoline-3-oxides (231) and (232), have been examined. ESR spectra of nitroxyl radicals containing one or two fluorine atoms at a-C have been registered (Scheme 2.108) (523). In the case of... 

For reviews dealing with chemistry of 2H- and 4H-imidazoles, see (a) Sammes MP, Katritzky AR. In Advances in Heterocyclic Chemistry, Katritzky AR (Ed.), Academic Press, New York, Vol. 35, 375-412, 1984 ... 

The reaction of a-halogenoximes 126 with amidines in the presence of iron carbonyls gives imidazoles 127 in 31-79% yields. The reaction occurred via deoxygenation of 4H-1,2,5-oxadiazines by iron carbonyls (equation 55). Efficiency of carbonyls decreased in the following order Fe3(CO)i2 > Fe2(CO)9 > Fe(Co)5 . ... 

One interesting difference between the known examples of 2H- and 4H-imidazoles is that nearly all in the latter group contain at least one hetero-linked substituent. This arises because the major synthetic approach is via imidazolone derivatives (see next section) a second important route using 1,2-diketones places an OH group at C-4, and electrophilic additions to 1//-imidazoles also tend to introduce heteroatoms at this site. 

On prolonged standing in acid solution, 83 is converted to 84 (Scheme 22). Salt 86, prepared readily from 85, ° can give 4H-imidazoles, including 87 and 88, on addition of nucleophiles. 

The H-NMR signal for the proton at C-4 in 88 (Scheme 26), the only 4H-imidazole known to be stable in this tautomeric form, is found at S 5.17. Only single examples of a base (79) and a cation (74) " with a proton at C-2 have had spectra recorded (Scheme 26) no data are available for protons at C-5. 

Ralph and Grunwald (68JA517) have used this method to determine that proton transfer from imidazolium ion (4H ) to imidazole (4) involves more than one water molecule. The rate of transfer of NH protons of (4H ) to water is fc = (1.07 + 0.11) X 10 sec ... 

The quatemization of 4//-imidazoles occurs at the 3-position (Scheme 65) (64AHC(3)i). 4H-pyrazoles are also readily monoquatemized. 

There are a number of thermal reactions which transform isoimidazoles into the fully aromatic compounds, e.g. the rearrangement of 2,2-dialkyl-2H-imidazoles (16S) to the 1,2-dialkyl isomers proceeds by a concerted process. In the absence of hydrogen sources thermolysis of 2,4,4-triaryl-5-methylthio-4H-imidazoles gives l,T-biimidazolyl products... 

Nucleophiles attack 2,4,5-trichloro-2H-imidazoles or 2,4-dichloro-4H-imidazoles with displacement of halogen (Scheme 79) (76CB1638). 

An interesting corollary of these reactions is the formation of 2-aminoimidazoles from guanidines and suitably substituted ketones. At -10 °C in methanol, l-phenylpropane-1,2-dione reacts with 1,1-disubstituted guanidines to give 2-(disubstituted amino)-4-hydroxy-4-methyl-4H-imidazoles (121) which, on catalytic hydrogenation, give excellent yields of... 

Cleanse of benzyl groups. On refluxing 5.5-dibenzyldithiohydantoin (1) and 4,4-dibenzyl-2,5-bis(methylthio)-4H-imidazole (2) with an aromatic hydrocarbon and... 

Selenium has been found to be an efficient hydrogen acceptor in the dehydrogenation of 2-aryl-2-imidazolines to 2-arylimidazoles. When 2,4,4,5-tetraphenyl-4H-imidazole is heated, it rearranges to the 1,2,4,5-tetraphenyl isomer. ... 

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